Ultrasound presents two challenges for the animal trying to hear it. First,
high frequency translates to short wavelengths; the hearing organ must be miniaturized
to match the wavelength. Second, high frequency sounds tend to be supported
by little energy. Not only do they dissipate rapidly as the sound travels, making
them relatively faint even close to the source, they also are subject to absorption
by the hearing organ without being transduced into a signal to the central nervous
system.

In order to accommodate the lower energy of ultrasound, the hearing membrane,
or tympanum, is typically thinner in animals which rely on ultrasound for communication
or navigation. The outer ears (pinnae) of mammals which perceive high frequency
sound may be quite complex; bat ears are characterized by grooves and channels
which help to carry sounds to the typmpanum, as well as maintaining small differences
in frequency (pitch) and amplitude (volume) which can be used to localize sound
sources.

Ultrasonic signals are produced in two contexts. First, in echolocation, an
animal (generally we think of bats doing this) produces high pitched sounds
which are reflected off objects in the bats flight path. The use of high pitched
sounds (ultrasound) has several advantages in echolocation. First, the short
wavelength of these sounds makes them more likely to bounce back to the bat,
rather than bend around the object. This, of course, is essential if echoes
are to be used in orientation. Second, it takes relatively little energy to
produce these sounds, and third, they dissiapate rapidly, reducing confusion
from "old" sounds that could still be bouncing around an area.

Second, ultrasounds are used by several kinds of animals in social contexts.
Bats use ultrasounds to communicate with mates, as do murid rodents (rats and
mice) and various sorts of moths. In addition, rodent pups use ultrasound to
call their mothers if they become isolated from her.

Because bats prey on insects, many insect species are attuned to bat echolocation
calls and take evasive measures if they hear a bat call. Males produce a calling
song to attract females in greater wax moths (Galleria mellonia) and
lesser wax moths (Achroia grisella); they stop calling if a calling
bat approaches; presumably the bat can orient to the mating call of the moth.